The present invention relates to an apparatus and method for controlling the common mode impedance for a circuit, conductor or cable without making a direct connection to the circuit, conductor or cable.
Many testing and design applications require establishing and controlling a stable, predictable common mode impedance for a circuit, conductor or cable with multiple conductors. A typical requirement in design applications is to match the impedance of a first circuit or conductor with that of a second circuit or conductor. In testing applications, one may wish to vary the impedance of a second circuit having an impedance ZL when coupled to a first circuit having an impedance Zin without varying the components of the second circuit.
A typical circuit for controlling the impedance or matching the impedance between a first circuit having an internal impedance Zin and a second circuit having a load impedance ZL is shown in FIG. 1. In FIG. 1, a series impedance Zseries and a parallel impedance Zparallel provide the necessary impedance control between the internal impedance Zin and the load impedance ZL. Either or both the series impedance Zseries and the parallel impedance Zparallel may be controllable to provide the desired impedance control. FIGS. 2A to 2H show a plurality of circuit constructions known in the art to provide impedance control between a first circuit and a second circuit by using variable inductors 21 and variable capacitors 23.
These prior art apparatus and methods require that one or more components be directly coupled between the first circuit and the second circuit. Such a coupling generally requires that the conductors between the two circuits be breached by, for example, cutting the conductors and wiring in the components for matching or controlling the impedance. Hence, prior art apparatus and methods for the insertion of impedance control elements requires one or more invasive connections.
Many testing and design applications require establishing and controlling a stable, predictable common mode impedance for a circuit, cable, or multiple conductor cable without making a direct, ohmic, or other invasive connection to the circuit, conductor, or multiple conductor cable. Further, these applications frequently require maintaining the impedance over a wide frequency range.
Therefore, there exists a need in the art for a method and apparatus for providing a stable, predictable, and wide frequency range impedance without making an invasive connection.
It is an object of the present invention to provide a method and apparatus for providing stable and predictable control over the impedance of a circuit, conductor, or multiple conductor cable without requiring a direct, ohmic, or invasive connection. It is a further object of the present invention to provide such impedance control over a wide frequency range. Another object of the present invention is to provide for impedance control over antenna structures.
In accordance with an aspect of the present invention, an embodiment of the present invention provides a method for controlling the input impedance of a circuit having one or more conductors and having an intrinsic impedance, the method comprising the steps of: disposing a portion of a conductor or conductors as a primary of a transformer; coupling a secondary of the transformer to a secondary impedance; coupling another portion of the conductor or conductors to ground with a capacitive impedance; and controlling the secondary impedance to provide the desired input impedance.
In accordance with another aspect of the present invention, an embodiment of the present invention provides an apparatus for controlling the input impedance of a circuit having one or more conductors and having an intrinsic impedance, the apparatus comprising: a transformer having a primary and a secondary, the primary having one or more primary turns and the secondary having one or more secondary turns, and at least one conductor being disposed as the primary; a secondary impedance coupled to the secondary; and a capacitive element coupling the conductor to ground.
In both the method and apparatus described above, the transformer may have a turns ration between the primary and the secondary of N, such that the desired input impedance Zin is given by Zin=N2Z1, where Z1 is the secondary impedance. The capacitive element may be provided by deploying a portion of the conductor closely proximate to a ground plane or coiling the conductor and the ground plane together. The capacitive element may also be provided by capacitive elements coupled between multiple conductors and a common core transformer, where the common core transformer is coupled to ground.
In accordance with another aspect of the present invention, an embodiment of the present invention provides a method for controlling the intrinsic impedance of a conductive antenna structure comprising the steps of: disposing a portion of the conductive antenna structure as a primary of a transformer; coupling a secondary of the transformer to a secondary impedance; and controlling the secondary impedance to provide the desired input impedance.
In accordance with another aspect of the present invention, an embodiment of the present invention provides an apparatus for controlling the intrinsic impedance of a conductive antenna structure, the apparatus comprising: a transformer having a primary and a secondary, a portion of the conductive antenna structure comprising the primary; and a secondary impedance coupled to the secondary of the transformer.
In both the method and apparatus described above, multiple secondary impedances may be coupled to the secondary of the transformer, where each secondary impedance may be separately selected by a switch. Each secondary impedance can then provide different operating characteristics for the antenna structure. The operating characteristics may be such to provide maximum effectiveness for the antenna structure with one selected secondary impedance and minimum effectiveness with another selected secondary impedance.